N,N′-Diarylformamidine Dithiocarbamate Ag(I) Cluster and Coordination Polymer

Abstract

An Ag(I)formamidine cluster Ag₆L1₆ (1) and an Ag(I)formamidine coordination polymer Ag₇(L2)₂ 2 (L1 = N,N′-bis(2,6-disopropylphenyl) formamidine dithiocarbamate and L2 = N,N′-mesityl formamidine dithiocarbamate) have been synthesized from the reactions of L1 and L2 with AgNO₃ respectively. The complexes were characterized using spectroscopic and analytical methods, including single-crystal X-ray diffraction. In the structure of 1, a six vertex distorted square bi-pyramidal octahedron is formed from an Ag₆ core. The N,N′-bis(2,6-disopropylphenyl) formamidine dithiocarbamate ligands stabilize this core through two main –CS₂ bridging modes giving a propeller like structure. In the structure of 2, each of the two Ag(I) centers are bridged by two N,N′-mesityl formamidine dithiocarbamate ligands forming 8-member Ag₂(CS₂)₂ metallacycles with an inversion center in the middle of the Ag—Ag argentophilic bond. The metallacycles are connected through Ag—S bonds forming ribbons in the crystallographic a-axis. The Ag(I) centers are coordinated to two N,N′-mesitylformamidine dithiocarbamates through the dithiocarbamate S atoms. The thermal decomposition of complexes 1 and 2 had similar thermograms with one major weight loss activity and the formation of elemental silver particles thereafter.

1. Introduction

Silver(I) complexes have been applied in various fields, therefore, attracting great attention due to their structural features and functional considerations [1,2,3]. They have been tested as fluorescent materials [4], semiconductors [5,6], source of nanomaterials [7] and biological imaging agents [8] due to their interesting and fascinating physicochemical properties. Ag(I) complexes have also shown potential medical applications, most especially as antibacterial [9,10], antifungal [11], anticancer [12], and antimalarial agents [13]. Argentophilic bonding interactions which exist between seemingly closed-shell silver(I) atoms center have been reported to be responsible for their significant physical properties as well as their structural details [2]. Structurally, they possess coordination geometries ranging from two-coordinate (linear) to eight-coordinate (tetragonal prism) [14]. Factors, such as reaction conditions, as well as alkyl substitution effects might be responsible for the formation of a variety of complexes with unprecedented structures [15,16]. In the past decades, monomeric, dimeric, hexameric and polymeric structures have been reported [17].

Dithiocarbamates (R₂CNS₂⁻) belong to a class of mono anionic 1,1-dithiolate ligands, and they are often prepared by nucleophilic addition reaction of primary or secondary amines and carbon disulfide in the presence of a base as a proton acceptor [18]. Dithiocarbamates have been reported to react with silver(I) ions to form Ag(I) complexes of various geometries [19,20] and also serve as efficient stabilizers for silver nanoparticles [21]. Silver(I) dithiocarbamates, [Ag(S₂CNR₂)]_n have been known as far back as the 1950s, but little work had been done on them probably due to their low solubility [17]. They have been widely used as a precursor to acanthite (α-Ag₂S), a potential material for microelectronics [22,23]. Early crystallographic studies revealed the hexameric nature of Ag-S₂CNR₂ (R = Et, Pr, and n-Bu) in the solid states. They have a polymeric chain structure, in which the silver atom is bonded in a distorted manner by three dithiocarbamate ligand, two acting as µ₂-bridges and one acting as a chelate [15,24].

Yin et al. reported the crystal structure of poly [(µ₃-N,N-dibenzyldithiocarbamato-κ⁴S,S’:S:S’) silver(I)] [17]. Each Ag(I) cation in this complex was bonded to two pairs of sulfur atoms from three N,N′-dibenzyldithiocarbamate ligands, conforming to distorted tetrahedral geometry. The crystal structure of hexakis (µ₃-N,N-diisopropyldithiocarbamato) hexasilver(I) had also been reported by Yin and co-workers [25]. The metal center of the complex was centrosymmetric, and its hexanuclear structure entails two cyclohexane-like Ag₃S₃ units which are joined together by the S atoms of the thiocarbamate groups with the Ag...Ag distances ranging from 3.0382(5) to 3.0985(5) Å. Herein, we report the synthesis, characterization and thermal analysis of silver(I) dithiocarbamate complexes derived from symmetrical N,N′-diarylformidine dithiocarbamate ligands.

2. Materials and Instrumentation

All solvents (ACS reagent grades ≥ 99.5%) were obtained from Sigma-Aldrich (Johannesburg, South-Africa) and used as purchased without further purification. Reagents: 2,6-diisopropylaniline (97%), 2,4,6-trimethylaniline (98%), triethyl orthoformate (99%), and carbon disulfide were also obtained from Sigma-Aldrich. AgNO₃ (98%) and KOH (85%) were obtained from Promark Chemicals, Johannesburg, South Africa.

¹H and ¹³C NMR spectra were recorded at 25 °C on a Bruker Avance^III 400MHz spectrometer (Karlsruhe, German) at the University of KwaZulu-Natal, Westville Campus. Both ¹H NMR and ¹³C NMR data were measured in parts per million relative to the residual solvent signals (chloroform-d for ¹H with δ = 7.26 ppm and ¹³C NMR δ = 77.00 ppm). IR spectra were obtained on a PerkinElmer Universal ATR spectrum 100 FTIR spectrometer (Shelton, CT, USA), UV-Vis absorption spectra were recorded on Shimadzu UV-Vis-NIR spectrophotometer (Kyoto, Japan), and Q series^TM Thermal Analyzer DSC/TGA (Q600) (Newcastle, CA, USA), was used to determine the stability of the complexes.

3. General Synthesis Methods

3.1. Preparation of Complexes

The synthesis of N,N′-bis(2,6-disopropylphenyl) formamidine dithiocarbamate and N,N′-mesityl formamidine dithiocarbamate salts have been reported in our previous work [26]. The complexes were synthesized by dissolving two equivalents of the potassium dithiocarbamate salts in 15 mL of acetonitrile into which a solution 1 equivalent of AgNO₃ in 5 mL of H₂O was added drop-wise with stirring for 30 min at room temperature. The resultant yellow solids were collected by filtration washed three times with diethyl ether, and dried in the oven at 50 °C.

3.1.1. Ag₆(L1)₆ 1

The reaction of L1 (0.3 g, 0.6 mmol) with AgNO₃ (0.05 g, 0.3 mmol) in a mixture of acetonitrile and water gave complex 1 as a yellow powder in 79% yield. m.p. 214–216 °C. ¹H NMR (CDCl_3, 400 MHz) δ (ppm): 1.04(d, 66H, J_H,H = 6.68 Hz, CH₃-CH), 1.15(d, 42H, J_H,H = 5.92 Hz, CH₃-CH), 1.21(d, 36H, J_H,H = 5.92 Hz, CH₃-CH), 2.70(m, 12H, J_H,H = 6.0 Hz, CH-CH₃), 2.81(m, 12H, J_H,H = 6.56 Hz, CH- CH₃), 7.06(s, 19, Ar-H), 7.16(d, 11H, J_H,H = 7.28 Hz, Ar-H), 7.35(t, 6H, J_H,H = 7.76 Hz, Ar-H), 9.38(s, 6H, -C(H)=N). ¹³C NMR (CDCl₃, 100 MHz) δ (ppm): 23.95, 24.09, 25.56, 27.47, 29.16, 123.04, 124.26, 124.54, 129.63, 138.76, 144.87, 151.65 and 214.51. IR υ (cm⁻¹): 2960, 1639, 1474, 1252, 982, 844, 481. UV-Vis (CHCl₃, λ_max, nm): 238 and 314.

3.1.2. Ag₂(L2)₂ 2

The reaction of L2 (0.3 g, 0.80 mmol) and AgNO₃ (0.07 g, 0.4 mmol) in acetonitrile furnished complex 2 as a yellow powder. Yield 64% Melting point 220–222 °C. ¹H NMR (CDCl_3, 400 MHz) δ (ppm): 1.99(s, 9H, CH₃), 2.11(s, 9H, CH₃), 2.22(m, 18H, J_H,H = 5.92 Hz, CH₃), 6.77(s, 3H, Ar-H), 6.87(s, 2H, Ar-H), 6.91(s, 1H, Ar-H), 6.97(s, 2H, Ar-H), 9.43(s, 2H, -CH=N). ¹³C NMR (CDCl₃, 100 MHz) δ (ppm): 17.84, 18.06, 18.58, 18.67, 20.67, 20.73, 20.76, 21.28, 127.54, 128.41, 128.73, 128.86, 128.99, 129.35, 132.18, 133.89, 134.51, 136.05, 138.16, 138.50, 142.49, 144.79, 147.24, 151.27 and 214.01. IR υ (cm⁻¹): 2916, 1634, 1476, 1246, 982, 845, 482. UV-Vis (CHCl₃, λ_max, nm): 238 and 312.

All the spectra for complexes 1 and 2 are given in the supplementary materials.

3.2. Single Crystal X-ray Diffraction

The structure refinement parameter, as well as the crystallographic data for complexes 1 and 2, are given in

Table 1. The summary of X-ray crystal data collection and structure refinement parameters for complexes 1 and 2.

	1	2
Empirical formula	C156H210Ag6N12S12	C40H46Ag2N4S4
Formula weight	3285.29	926.79
Crystal system	Trigonal	Triclinic
Space group	R-3: H	P-1
a/Å	30.0941(6)	8.1650(5)
b/Å	30.0941(6)	15.5778(9)
c/Å	50.6555(11)	16.2365(10)
α/°	90	77.7000(10)
β/°	90	87.3590(2)
γ/°	120	83.1580(3)
Space group no.	148	2
Volume/Å3	39,730(2)	2002.9(2)
Z	9	2
ρcalcg/cm3	1.236	1.537
μ/mm−1	0.841	1.220
F(000)	15,336	944
Crystal size/mm3	0.320 × 0.210 × 0.140	0.32 × 0.19 × 0.11
2θ range for data collection/°	1.614 × 26.389	1.655 to 26.999
Index ranges	−31 ≤ h ≤ 37	−9 ≤ h ≤ 10
	−37 ≤ h ≤ 24	−19 ≤ k ≤ 18
	−62 ≤ l ≤ 61	−20 ≤ l ≤ 12
Reflections collected	91,820	6320
Independent reflections	17,643	8656
Data/restraints/parameters	17,643/0/862	8657/7/1474
Goodness-of-fit on F2	0.935	0.983
Final R indexes [I > = 2σ(I)]	R1 = 0.0409 wR2 = 0.0868	R1 = 0.0610 wR2 = 0.110
Final R indexes [all data]	R1 = 0.0869, wR2 = 0.1009	R1 = 0.0610, wR2 = 0.1196
Largest diff peak & hole (e Å−3)	1.597 and −0.799	1.551 and 0.879

. Evaluation of crystals and collection of data was done on a Bruker Smart APEXII diffractometer with Mo Kα radiation (I = 0.71073 Å) equipped with an Oxford Cryostream low-temperature apparatus operating at 101 K for all samples. Reflections were collected at different starting angles, and the APEXII program suite was used to index the reflections [27]. Reduction of data was carried out using the SAINT software [28], and the absorption corrections and scaling were applied using the SADABS multi-scan technique [29]. The two structures were solved by the direct method using the SHELXS program and refined using the SHELXL program [30]. Graphics of the crystal structures were drawn using Mercury software [31]. Non-hydrogen atoms were first refined isotropically and then by anisotropic refinement with the full-matrix least square method based on F² using SHELXL. All hydrogen atoms were positioned geometrically, allowed to ride on their parent atoms, and refined isotropically. The crystallographic data and structure refinement parameters for complexes 1 and 2 are given in Table 1.

4. Results and Discussion

4.1. Synthesis of N,N′-Diarylformamidines Dithiocarbamate Ag(I) Complexes

Complexes 1 and 2 were synthesized by reacting acetonitrile potassium salt solutions of L1 and L2 (Figure 1) with aqueous solutions of AgNO₃ in a 2:1 ratio. Both complexes were obtained as thermally stable yellow solids with melting points ranging between 214 and 222 °C, 1 having a lower melting point than 2. Both complexes are soluble in dichloromethane, chloroform, toluene and benzene.

4.2. Spectroscopic Studies

The ¹H and ¹³C NMR spectra for complexes 1 and 2 obtained in chloroform had signature peaks for the methane proton of the L1 and L2 as confirmed using their 2D NMR spectra. The azomethine proton of 1 and 2 was observed at 9.38 and 9.43 ppm in the spectra of L1 and L2, respectively, an upfield shift from 10.15 and 9.92 ppm [26]. The signals of aliphatic protons in the spectra of the complexes shifted noticeably downfield. For example, methyl protons in 2 appeared at 1.94, 2.03, and 2.15 ppm but were at 1.99, 2.11, and 2.22 ppm in L2. The downfield shift is a result of the drifts of electron density towards the metal ion center [32,33]. There were similar observations in the ¹³C-NMR spectra of 1 and 2, and an upfield shift of the carbon atom of -NCS₂ moiety to 214.15 and 214.01 ppm from 220.94 and 218.95 ppm in the spectra of L1 and L2, corroborating coordination of the S atom to Ag.

The FT-IR spectra of complexes I and 2 showed a strong absorption band at 1474 and 1476 cm⁻¹, stretching bands for the C—N bond of the thiouride group. These stretching frequencies are intermediate of a typical C—N single bond (1250–1360 cm⁻¹) and a double bond (1640 cm⁻¹), an indication of a partial double bond character of the thiouride bond [34,35]. The two vibrational bands around 1252–1246 cm⁻¹ and 982–983 cm⁻¹ in the spectra of 1 and 2 are assigned to the asymmetric C—(S)—S and the symmetric C—(S)—S moiety confirming the asymmetric linking of the sulfur atoms to the silver atoms [36]. The vibrational bands for υ(C=Nstr) of the azomethine (C(H)=N) in the formamidine backbone of the complexes were observed around 1634–1639 cm⁻¹.

In the electronic spectra of 1 and 2, two bands were observed at 238 and 312, and 314 nm and these are attributed to intraligand π→π* transition associated with N—C=S and π→π* transition within S—C=S groups of the coordinated dithiocarbamate ligands [37].

4.3. X-ray Structural Analysis

Suitable crystals for single-crystal X-ray diffraction analysis were obtained by slow diffusion of hexane into a chloroform solution of complexes 1 and 2, each. While complex 1 is an Ag(I) diarylformamidines dithiocarbamate cluster, 2 is an Ag(I) diarylformamidines dithiocarbamate coordination polymer. Complex 1 is a polymorph of the previously reported structure of Ag₆[CS₂(2,6-ⁱPr₂C₆H₃NC(H)=NC₆H₃-ⁱPr₂)]₆ which crystallized in the Ia3 cubic space group (

Table 2. Comparison of some selected X-ray crystal data collection and structure refinement parameters for complex 1 and its polymorph.

	1	Polymorph
Empirical formula	C156H210Ag6N12S12	C174H262Ag6N12S12
Formula weight	3285.29	3553.88
Crystal system	Trigonal	Cubic
Space group	R$\overline{3}$	Ia$\overline{3}$
a/Å	30.0941(6)	32.283(4)
b/Å	30.0941(6)	32.283(4)
c/Å	50.6555(11)	32.283(4)
α/°	90	90
β/°	90	90
γ/°	120	90
Volume/Å3	39,730(2)	33,644.8(2)
Z	9	8
ρcalcg/cm3	1.236	1.403
Largest diff. peak & hole (e Å−3)	1.597 and −0.799	1.551 and 0.879

). The previously reported structure was synthesized by the insertion of CS₂ into a dinuclear Ag(I) complex of Ag₂[2,6-ⁱPr₂C₆H₃N)₂C-(H)]₂ using toluene as a solvent for the reaction [38].

Complex 1 is assembled through six Ag(I) centers and six diarylformamidines dithiocarbamate ligands (1:1 ratio). The asymmetric unit contains one-half of the cluster and is made up of three Ag(I) centers, coordinated through bridging by three diarylformamidines dithiocarbamate molecules (Figure 2a). The other half is generated through an inversion center. In the crystal, a six-vertex distorted square bipyramid octahedron is formed in which Ag3 forms the apexes of the pyramid while Ag1₂Ag2₂ (red dashed lines in Figure 2b) for the base of the bipyramid (Figure 2b). The square bipyramid is stabilized by six L1 ligands which have the coordination modes μ¹- μ¹- for four of the ligands while for the other two have a μ¹- μ²- coordination mode. With the CS₂ moieties added on, a propeller like core is formed (Figure 2c).

Comparison of 1 to Ag₆[CS₂(2,6- ⁱPr₂C₆H₃NC(H)=NC₆H₃- ⁱPr₂)]₆

The literature structure is solvated with one molecule of chloroform, while 1 is not. The Ag(I) octahedron core of the literature structure forms a perfect square bipyramid, whereas that of 1 is slightly distorted. The Ag—Ag distances in 1 range between 2.9204(4) and 3.3452(4) Å while the distances are between 3.0001(3) and 3.3178(3) Å for literature structure. In both complexes, the Ag—Ag argentophilic distances are all less than the sum of the van der Waals radii of two Ag atoms, 3.44 Å [23,24].

Each Ag(I) center in 1 is coordinating to three sulfur atoms in a κ¹κ¹κ¹-S fashion, a geometry around Ag(I) that can be described as distorted trigonal pyramidal in which the Ag(I) center serves as the apex of the pyramid. The S—Ag—S bond angles range between 107.07(4) and 123.40(3)° in 1 while it ranges between 109.29(3) and 127.14(3) in the literature structure. The Ag—S bond distances are 2.4857(11), 2.5231(10), and 2.5322(9) in comparison to 2.4779(3) and 2.5008(2) for the literature structure.

In the crystal of complex 2, the asymmetric unit consists of two diarylformamidines dithiocarbamate ligands (L2) and two Ag(I) centers (Figure 3a). The Ag(I) centers are bridged by two diarylformamidine dithiocarbamate units in such a way that two Ag1s are paired and likewise two Ag2s. In doing so, each bridged Ag(I) pairs with an 8-member bimetallocycle, and an inversion center is formed. The metallacycles are connected through two centrosymmetrically related Ag—S bonds leading to a ribbon that runs in the a-crystallographic axis (Figure 3b).

Just like in 1, the Ag(I) center is coordinated to three S atoms giving a trigonal pyramidal geometry around the metal center. This differs from the coordination polymer of [Ag(dibenzyldithiocarbamate)]∞ [17] where each Ag(I) cation is bonded to two pairs of sulfur atoms from three dithiocarbamate ligands resulting in a distorted tetrahedral geometry. The S—Ag—S bond angles range from 102.58(3)°—129.07(4)° and these are similar to the ones for coordination polymer of [{Ag(diethyldithiocarbamate)}₃]∞ which ranges from 109.49(4)°—128.56(4)° [24].

The Ag—S bond distances are in the range of 2.4757(10)–2.5716(12), i.e., Ag(2)—S(1), Ag(2)—S(2), and Ag(2)—S(3) are 2.5716(12), 2.4757(10), and 2.5660(10) respectively (

Table 3. Selected bond length (Å) and angles (°) for complexes 1 and 2.

Bond distances (Å)	1	Polymorph	2
Ag—Ag	2.9545(4)	3.0001(1)	2.8829(6)
Ag—Ag	2.9695(4)	3.0001(1)	3.1932(5)
Ag—Ag	3.3452(4)	3.0001(1)	3.0644(5)
Ag—S	2.4857(11)	2.4777(1)	2.5029(9)
Ag—S	2.5322(9)	2.5007 (1)	2.4805(10)
C—S	1.681(4)	1.6962(1)	1.682(5)
C—S	1.739(4)	1.7404(1)	1.723(4)
C—N	1.362(5)	1.3632(1)	1.366(4)
C—N	1.371(4)	1.4639(1)	1.406(5)
Bond angles (°)
Ag—S—Ag	85.03(3)	82.20	74.38(3)
S—Ag—S	107.07(4)	109.29	111.97(4)
Ag—Ag—S	77.81(2)	80.27	88.90(2)

) and comparatively, they are similar to those previously reported related complexes [39,40]. The Ag—Ag bond lengths in complex 2 are identical to those observed in 1. The C—S bond distances in the dithiocarbamate ligand (L2) backbone of complex 2, fall between ideal single and double C—S bonds indicating partial delocalization of π-electron density over the entire S₂CN fragments in the complex [26,41].

4.4. Thermal Decomposition Studies

The thermal decomposition of the two silver complexes was examined by thermogravimetric (TG) analysis, and the superimposed TG graphs are represented in Figure 4. It is observed from the TGA curve that the thermal decomposition of the complexes shows one step that is equivalent to about 78% weight loss associated with the decomposition of the dithiocarbamate backbone ligand and the dominant weight loss of complex 1 occur in the temperature region of 250–325 °C while the one for 2 occurs at 250–350 °C. In both complexes, there is almost no weight loss below 100 °C and above 350 °C. The decomposition of 1 and 2 led to the formation of elemental silver residue with about 22% residue, which was about 2% more than the calculated value of 20% for 1 and 1% less than the calculated value of 23% for 2.

5. Conclusions

Conclusively, Ag(I)formamidine cluster and Ag(I)formamidine coordination polymer has been synthesized and characterized by means of thermogravimetric analysis together with FT-IR, UV-vis, ¹H, and ¹³C NMR spectroscopy. X-ray structural analysis showed that the Ag₆ core of structure 1 formed a six vertex distorted square bi-pyramidal octahedron while in structure 2, each of the two Ag(I) centers are bridged by two N,N′-mesityl formamidine dithiocarbamate ligands to form an 8-member Ag₂(CS₂)₂ metallacycles with an inversion center in the middle of the Ag—Ag argentophilic bond. The thermal analysis of complexes 1 and 2 led to the formation of the elemental silver residue.